An ad hoc network is one type of a self-configurable network linked by radio communication. An ad hoc network is composed of plural nodes. The nodes in an ad hoc network transmit and receive packets via multi-hop communication. The multi-hop communication is a technique in which nodes not existing in mutual communication areas perform communications via different nodes existing in the respective communication areas.
One technique utilizing the ad hoc network is a system in which radio-communicable nodes are built in house power meters so that a service such as meter reading is performed via the ad hoc network without a worker actually going to the site. In the ad hoc network handling personal information such as household power consumption, secure communications are required to be performed from the viewpoint of privacy, tampering, etc. Thus, the conventional system ensures secure communications by encrypting packets transmitted and received between the nodes in the ad hoc network (see, for example, Japanese Laid-Open Patent Publication Nos. 2003-348072, 2010-98597, 2007-88799, and 2009-81854).
Even if packets are encrypted, a proper packet transmitted on the network in the past may be intercepted by an attacker in the ad hoc network, by which the attacker can resend the intercepted proper packet on the ad hoc network to launch an attack (resend attack) of congesting the network. Thus, to ensure network communication quality, the ad hoc network needs to provide against a resend attack.
In a conventional system, a node transmitting a packet, stores a packet transmission time into the packet. A technique exists in which a node receiving the packet compares the time therein with the transmission time indicated in the received packet and, if the two times are far apart, regards the packet as being a resend attack and discards the packet.
Nonetheless, with the conventional technologies above, the times of all nodes on the ad hoc network have to be synchronized. For example, a specific node in the ad hoc network or in a gateway serving as a relay device with another network broadcasts a time synchronizing packet periodically to thereby implement time synchronization among the nodes in the ad hoc network.
Accordingly, an ad hoc network performing meter reading transmits and receives the time synchronizing broadcast packet in addition to meter reading data packets, resulting in an increased communication load on the ad hoc network.